« " - nsTiThis report was prepared as an account of woi'- sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi- C O N F - 8 5 1 1 1 7 2 9bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer- nRfifi 0 Q 8 8 5 3ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or tin )ly its endorsement, recom-mendation, or favoring by the United States Government or an; agency thereof. The viewsand opinions of authors expressed herein do no! necessarily state or reflect those of theUnited States Government or any agency thereof

ANALYSIS OF GASEOUS-PHASE STABLE AND RADIOACTIVE ISOTOPES

IN THE UNSATURATED ZONE, YUCCA MOUNTAIN, NEVADA

la C. Yang1, Herbert H. Haas2, Edwin P. Weeks1, Donald C. Thorstenson3

1U.S. Geological SurveyDenver, Colorado

2Institute for the Study of Earth and Man,Southwestern Methodist University,

Dallas, Texbs3U.S. Geological Survey,

Reston, Virginia

Abstract

MASTERThe Nevada Nuclear Waste Storage Investigations Project of the U.S.

Department of Energy provides that agency with data for evaluating vol-canic tuff beneath Yucca Mountain, Nevada, to determine its suitabilityfor a potential repository of high-level radioactive waste. Thickness ofthe unsaturated zone, which consists of fractured, welded and nonweldedtuff, is about 1,640 to 2,460 feet (500 to 750 meters). One question tobe resolved is an estimate of minimum ground-water traveltime from thedisturbed zone of the potential repository to the accessible environment.Another issue is the potential for diffusive or convective gaseous trans-port of radionuclides from an underground facility in the unsaturated zoneto the accessible environment.

Gas samples were collected at intervals to a depth of 1,200 feetfrom the unsaturated zone at Yucca Mountain, Nevada. Samples wereanalyzed for major atmospheric gases; carbon dioxide in the samples wasanalyzed for carbon-14 activity and for 613C; water vapor in the sample*was analyzed for deuterium and oxygen-18. These data could provideinsight into the nature of unsaturated zone transport processes.

Two different collection t*»chni<]ues and analytical methods were usedfor determination of C02 concentration, carbon isotopes, and water vapor.The results indicate that C0» concentrations measurpri by both methods :

were not consistent, possibly because of continual fluctuations of uahaowttorigin. Data for 14C indicate general agreement between the molecularsieve and K0H methods for comparable samples, and significantly ff

OlSTWBUTIMi tf rift, 0 8 W I W T IS

results for 613C sampler. More data are needed to resolve the differ"ences. The majority of post-bomb 14C was observed to a depth of 40 feet(12.2 meters) below surface, and less than pre-boab activity was measuredat and below a depth, of 60 feet (18.3 meters). Data for d l s0 y indicateexchange of oxygen atons between the water and the molecular sieve occurredduring degassing and affected the oxygen-isotope values. Data for 6Dseen to be unaffected by- exchange processes. Values of 60. are close tothose in ground water at Yucca Mountain and at nearby areas, althoughvalues of 6D. determined by molecular-sieve and cold-trap methods arenot consistent in direction or magnitude.

Introduction

The Nevada Nuclear Waste Storage Investigations (NNWSI) Project ofthe U.S. Department of Energy provides that agency with data for evaluatingvolcanic tuff beneath Yucca Mountain, Nevada, to determine its suitabilityfor a potential repository of high-level radioactive waste. This investi-gation is being conducted in cooperation with the U.S. Department of Energy,Nevada Operations Office, under Interagency Agreement DE-AI08-78ET44802.

One of the questions to be resolved is an estimate of minimum ground-water traveltime from the disturbed zone of the potential repository tothe accessible environment. The U.S. Nuclear Regulatory Commission's(1983) siting criteria state that a site would be favorable if the pre-waste-emplacement ground-water traveltime from the disturbed zone to theaccessible environment substantially exceeds 1,000 years. The U.S.Department of Energy's (1984) siting guidelines on this issue state that asite would be disqualified if the expected pre-waste-emplacement ground-water traveltime from the disturbed zone to the accessible environment isless than 1,000 years; a site weld be favorable if ground-water travel-time is more than 10,000 years. Another question is the potential fordiffusive or convective transport of gaseous radionuclides from an under-ground facility in the unsaturated zone to the accessible environment.The U.S. Nuclear Regulatory Commission's (1984) proposed rule on thisissue states that a site would have a potentially adverse condition ifthere is a risk of vapor transport (radioactive gases) from the under-ground facility, located in the unsaturated zone, to the accessibleenvironment.

The concept of storing nuclear waste in the arid-clirrate vadose zoneappeared in the literature more than a decade ago (Winograd, 1972, 1974),and in more recent papers by Winograd (1981) and Roseboom (1983).However, little detail exists of the hydrogeology of Yucca Mountain. Noisotopic and chemical data have been reported for the unsaturated-zonewater of Yucca Mountain. The purpose of this study is directed atevaluating analytical methods for determining gas constituents in theunsaturated zone of Yucca Mountain.

489

Physiographic and Geologic Description of Yucca Mountain

The Yuccs Mountain study area is located near the southwesterncorner of the Nevada Test Site in southern Nevada, within the Great Basinphysiographic province, and in the most arid region of the United States.Average annual precipitation at the mountain is estimated to be aboutS.9 in. (150 mm), and the average annual recharge rate is estimated torange from 0.02 to 0.18 in. (0.5 to 4.5 mm) (Montazer and Wilson, 1984).The altitude of the mountain is 4,823 ft (1,470 m). A very thick, 1,640to 2,460 ft (500 to 750 m), unsaturated zone occurs beneath Yucca Mountain.The zone consists of extensively fractured, densely welded., minimallyporous, but transmissive, ash-flow tuff, interbedded with less fractured,nonwelded, porous, but less transmissive, argillic and zeolitic beddedtuff and ash-flow tuff. The mountain consists of a series of north-trending fault-block ridges with an eastward tilt of about 5° to 30°. Theproposed high-level nuclear-waste repository would be emplaced in thedensely welded fractured tuff in the central block of Yucca Mountain.

Sample Collection

Gas samples were collected from the borehole USW UZ-1 located atthe northern end of the central block on Yucca Mountain (fig. 1). Bore-hole USW UZ-1 was drilled by a reverse-vacuum method to a depth of 1,269 ft(387 m) in 1983 (Whitfield, 1985, this proceedings).

During drilling, air was withdrawn through the inner string of theconcentric dual-string pipe by vacuum at a rate of about 1,400 ft3/min(39.6 m3/min). Air was injected into the annulus of the inner stringat a rate of about 315 ft3/min (8.9 m3/min) and air also circulateddownward through the annulus between the borehole wall and the drill-string to make up the difference between the injected and withdrawnvolumes. Sulfur hexafluoride (SF6) was added to the injected air or tothe air stream in the hole annulus or to both at a rate of 40 to 50 mL/minexcept for occasional increases to a rate of 100 mL/min or more. Ifperfect mixing is assumed, the resulting mixture should have containedabout 1 ppmv (part per million by volume) SF6- However, the vacuum ratevaried somewhat, particularly when the inner string became partly plugged,and may have decreased to a few hundred cubic feet per minute at times.At those times, the SF6 concentrations may have increased to as much as afew paius per million by volume.

The hole was completed with 15 sampling stations, including gasprobes, isolated from each other by silica flour and cement grout.Details of probe construction are given by Montazer et al. (1985, thisproceedings).

Samples for Analysis of Sulfur Hexafluoride

Samples were collected and analyzed for SF6 onsite on March 31-April *i1984, by personnel of Tracer Research, Inc., Tuscon, Arizona, using a gas

chromatograph equipped with an electron-capture detector. In addition,later samples were collected and shipped to Tracer Research, Inc., f°r :analysis. For the samples collected aud analyzed on March 31, each probe

•;•$

490

-; NEVADA- TEST

SITE

CENTRALBLOCK

0 15 KM

0 5 Ml

Figure 1. Location of borehole USW UZ-1

491

was pumped for 30 miu (more than 30 times the volume of probe) prior tocollecting a sample for analysis. Samples were collected through siliconetubing by syringe and injected directly into the gas chromatograph. Aftersampling was completed on March 31, probes 1-13 were connected throughmanifolds to two rotary-vane pumps and pumped overnight. The probes wereresampled on April 1, the sampled probes being pumped individually, usinga peristaltic pump for about 15 min prior to sampling. The deeper probeswere pumped overnight on April 1 and again resampled on April 2.

Samples for analysis of SF6 also were collected on September 17,1984. These samples were collected in previously evacuated stainless-steel containers and shipped to Tracer Research, Inc., for analysis.

Samples for Analysis of Carbon Dioxide

Samples were collected for analysis of C02 by gas chromatography(thermal-conductivity detector) in November 1983; January, March, April,September, and December 1984; and May 1985. In addition, gas sampleswere analyzed onsite for C02 by gas chromatography on April 1-2, 1984.Concentrations of C0 2 also were computed for March and May 1985 fromthe volume of CO2 recovered during molecular sieve trapping, as describedbelow, divided by the volume of soil gas pumped through the molecularsieve.

For the gas-chromatography method, gas samples were collected in15.25-in.3 (250-mL) flow-through gas collector (glass tube), with stop-cocks on both ends. For those samples not collected for 1 4C0 2 analysis,the gas collector was installed upstream of a rotary-vane vacuum pump,which was operated for 30 min before the sample was collected. Afterthat time, the downstream stopcock was closed and the pump was shut off.After 5 min, the upstream stopcock was closed. The stopcocks were tapedto insure against accidental opening during shipping, and were shipped tothe U.S. Geological Survey at Reston, Virginia, for analysis. For samplescollected for 1 4C0 2 analysis, a similar procedure was followed, exceptthat a peristaltic pump was used.

Samples for Analysis of Carbon Isotopes

Samples were collected for the analysis of 14C02 activity and fordetermination of 13C/12C ratios in April 1984 and in March and April-May,1985. In 1984, samples were collected from probes 1-13 by trapping withKOH solution, using a method described by Haas et al. (1983). In 1985,samples were collected using molecular-sieve traps for all 15 probes andby KOfi" for 5 probes.

The KOH method: For this method, described in detail by Haas et al.(1983), soil gas is pumped through KOH solution by a peristaltic pumpinstalled downstream from the bubbler. The soil gas is dispersed througha fritted plate into 15.25 in.3 (250 mL) of 5.0 molar KOH solution at • ;..rate of 30.5 in.3 (500 mL/min), and the C02 is absorbed in the solution. ..-£•<Ideally, about 48.8 in.3 (800 mL) of C0 2 is trapped. However, C02 concen- .v'tratiotiis were so small in the soil gas from all probes except probe 1 •_;.-.that taey were pumped for 10 d to obtain about 4.9 to 18.3 in.3 (80 t0 ,̂;300 ml) for analysis. U&.

?•••

492

Molecular-sieve (MS) method: Gas-collection cylinders with a volumeof 18.3 in.3 (300 cm 3), filled with Linde 5A molecular-sieve pellets,1/16 iu. (0.159 cm), were used for gas sampling. Details of the samplerare shown in figure 2a. The cylinder was made of 30AL stainless steel,connected to Nupro1 B4HK2 valves attached to hosebarbs at both ends. Inaddition, one end of the cylinder was equipped with a check-valve betweenthe hosebarb and valve to prevent backflow of air during sample collection.

NUPRO B4HK2VALVE -

GLASS WOOL GLASS WOOL

NUPRO B4HK2METAL VALVE

HOSEBARB\

HOSEBARB

CHECK VALVESTAINLESS-STEEL CYLINDER CONTAINING LINDE 5A

MOLECULAR-SIEVE PELLETS

Figure 2a. Gas sampler

The onsite setup of equipment for soil-gas collection by the MSmethod is shown in figure 2b. Soil-gas probes were pumped overnightbefore sample collection to purge the lines and probes of any atmos-pheric air that might have been introduced while connecting the pumps tothe system. During sample collection, the sample gas was pumped at aflow rate of 30.5 in.3/min (500 m7,/min). The date, start, and end timesof pumping were recorded, so that total volumes of gas flow through thesampler could be calculated. The MS trapped both water vapor and CO2.If saturation of the MS were to be attained during the process, watervapor would displace C02• Therefore, a small ascarite-filled glass tubewas attached to the downstream side of the system to monitor C02~gasescape (formation of white NaHC03 on the ascarite). The uncertainty inthis gas detection was about 5 mg C02. A sampler of this size shouldtrap about 0.84 ft3 (24 L) of C02 at 25°C and 1 atmosphere in the absenceof water, and about 100 mL of liquid water. This capacity is sufficientfor C0 2 sampling at the Yucca Mountain site. If gases with substantialmoisture content are expected, a larger capacity cylinder, 30.5 in.3

(500 mL), needs to be used. The collection efficiency and performancedata are given in a paper by Yang (1983). Limited equipment and spaceallowed sampling only 5 to 8 probes at a time, with each sample requiring4 to 5 d to collect. The 15 probes were sampled in 2 onsite trips withprobes 9, 11, and 15 being resampled.

*Any use of trade name is for descriptive purposes only and does notconstitute endorsement by the U.S. Geological Survey.

493

ASCARITE

SIUCONE TUBING

GAS SAMPLER

PERISTALTIC PUMP

FLOW METER

SOIL GASPROBE

Figure 2b. Onsite apparatus for soil-gas collection.

Samples for Analysis of Water Vapor

Dry-ice acetone trap: For samples collected for analysis of watervapor, a dry-ice acetone slush was used. A glass-vacuum trap, immersedinto the slush contained in the dewar, replaced the gas t.ampler in figure2b. Collection time of 36 to 48 h yielded about 0.61 in.3 (10 mL) ofliquid water.

MS method: Water vapors and CO2 were collected simultaneously. TheMS was heated under vacuum at 350°C overnight prior to collection, toensure that no moisture from previous collections was trapped in thesieve. Collection efficiency was not determined.

Sample Analyses

Sulfur-Hexafluoride Samples

Sulfur-hexafluoride (SF6) samples were analyzed both onsite and inthe laboratory by personnel of Tracer Research, Inc., Tuscon, Arizona,using a gas chromatograph equipped with an electron-capture detector.

Carbon-Dioxide Samples

Carbon-dioxide samples collected by the gas collector were analyzed T;gas chromatography (thermal-conductivity detector) at the U.S. Geologic*Survey Laboratory in Reston, Virginia. Concentrations of CO2 determineby the MS method were computed from the volume of C02 recovered duringmolecular-sieve trapping divided by the volume of soil gas pumped tbrougothe sieve.

494

Carbon-Isotope Samples

KOH method: The C02 collected in the KOH solution was released byacidification and converted into beneene for 14C analysis by scintilla-tion counting at the Southern Methodist University (SMU) radiocarbonlaboratory. Details of the method were described by Haas et al. (1983).The ratio of l3C/l2C also was determined at the SMU isotope laboratory.For small-size samples (less than 15.25 in.3 or 250 mL C02 gas at 25°Cand 1 atmosphere), 14C concentrations were measured using the Tandemaccelerator mass spectrometer at the University of Arizona.

MS method: The C02 gas trapped in the MS was degassed at the U.S.Geological Survey laboratory in Denver. Because both C02 and H2O weretrapped in the MS, a system was designed to separate the two components;this system is shown in figure 3- The MS was heated to 35O°C in a vacuum

VACUUM ANDPRESSURE GAUGE

VACUUM GAUGE

GASSAMPLER

TO VACUUMPUMP

H2O

TRAP

CO2

STORAGECYLINDER

co2TRAP

Figure 3. Degassing system

1

system. Water was collected in a dry-ice acetone t rap ; CO2 was col lectedin a l iquid-ni trogen t rap. Complete degassing took 10 h or longer. Thevolume of the collected CO2 was measured in a ca l ibra ted flask equippedwith a pressure gage. The gas was then t ransferred to a storage cylinder.Carbon-dioxide samples of more than 36.6 i n . 3 (600 mL) at 25°C and1 atmosphere were; sent to Geochron Laboratory (Krueger Enterpr ise , Mass.)for I 4 c analyses, by conventional gas counting of CH4. Two samples withvolumes of 30.5 and 24.4 i n . 3 (300 and 400 mL) were dated at the SMUradiocarbon laboratory. Even smaller samples have been submitted to the

495

University of Arizona (Tucson) for analyses by using the Tandem acceleratormass spectrometer. Results were not available at the time of preparationof this paper.

Water-Vapor Samples

Water samples collected by cold trap and by MS (degassed, as men-tioned in the previous section) were sent to the U.S. Geological Surveylaboratory in Reston, Virginia, for determination of 3H, and 180/160 andD/H ratios.

Results and Discussion

Sulfur-Hexafluoride Concentrations

The results of the SF6 analyses are given in table 1. Based on theintroduced atmospheric air having an SF6 concentration of 1 ppmv, soilgas would contain 10 to 30% residual atmospheric air. However, themeasured C02 concentrations and carbon-isotope data indicate that theproportion of residual atmospheric ajir should be significantly less.Hence it is likely that the effective concentration of SF6 in the drill-ing air was greater than 1 ppmv, and may have been as much as 2 ppmv,resulting in a contamination level of 5 to 15% as of September 1984. Aircontamination should be successively less during subsequent sampling.

Carbon-Dioxide Concentrations

Concentrations of CO2 measured at borehole USW UZ-1 are given intables 2a and 2b. Concentrations were large in probe 1, which is inalluvium. Such concentrations are typical of soil gas at other sites(Thorstenson et al., 1983, for example), and results from root respirationand microbial activity in the soil zone. However, at probe 2, located innonwelded tuff a few feet below its contact with alluvium, and in alllower probes, concentrations were much smaller, and at several screens,concentrations were less than atmospheric.

The C02 concentrations also show substantial variations with tinesince sampling began in November 1983. The November 1983 samples areparticularly anomalous, possibly because they were collected only several -days after the hole had been instrumented, and C02 in the soil gas nayhave been sorbing and equilibrating with the stemming materials. How-ever, large variations in C02 concentrations with time were measuredeven after large volumes of soil gas were pumped for 14CC>2 sampling i°April 1984. Moreover, the trends are inconsistent, with concentration*;of samples from some probes alternately increasing and decreasing; °™*^samples showed trends of steadily increasing concentrations, and stii* .others showed steadily decreasing concentrations. Such variations "***]depth were unanticipated, and generally are net detected at other *ac*tions (Thorstenson et al., 1983, for example). Continued samplinghelp determine the causes of the variations, or concentrations oaf-eventually stabilize.

496

Table 1. Summary of SFs data for borehole USW UZ-1 March 31-April 2, 1984, and for September 17, 1984

[pprav = parts per million by volume]

ProbeNo.

Date

s Concen-tration(ppmv)

ProbeNo. Date

SF6 Concen-tration(ppmv)

3/314/19/17

1.47.60.34

3/314/19/17

3/314/19/17

3/314/19/17

3/314/19/17

3/314/19/17

3/314/19/17

.08-.58

M.OlS

.49

.30

.27

.58

.33

.26

.18

.16

.09

.13

.12

.09

.18

.15

.08

10

11

12

3/314/14/29/17

3/314/14/29/17

3/314/14/29/17

3/314/14/29/17

3/314/14/2

3/314/14/2

0.16.16.18.16

.19

.16

.15

.19

.15

.06

.07

.07

.15

.06

.07

.12

.25

.19

.19

.25

.18

.18

:Bad sample (?).

Concentrations of CO2 also were msamples. These should, assuming 100'integrated values for a large volume csamples from a given probe also indiracentrations, varying by as much as £ :later gas chrotnatography data, part::_ranges from good to poor. When the ;:poor, the results of the MS analyses 1the GC analyses.

eas.red in the molecular-sieveirspriag efficiency, representf s:il gas. However, separate*e: large variations in C0 2 con-c:;~.z of 5. Agreement with thelirly Zecer.ber 1984 and May 1985,rr?spicde^ce between methods isre ::r.3istently less than those of

Table 2a. Carbon-dioxide concentrations from soil gas bymolecular-sieve method, borehole USW UZ-1

[CO2 concentrations are percent (%) by volume; dashes indicate no data]

ProbeNo.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Depth

(ft)

42

62

93

131

201

266

348

421

501

621

747

871

998

1,100

1,207

Carbon Isotopes 1

(m)

12.8

20.3

28.3

39.9

61.3

81.1

106.1

128.3

152.7

189.3

227.7

265.5

304.2

335.3

367.9

:14C and

Collectiondate(1985)

—

--

--

-~

3/26-27

3/23-24

3/25-263/27

3/253/26

3/23-243/27

3/19-213/23

3/19-203/21

3/19-213/24-25

3/19-213/22-233/23-25

3/19-21

13C/12C ratio)

C02

(%)

--

--

--

--

--

0.030

.016

.064

.052

.011

.007

.014

.053

.011

.010

.016

.009

.004

.002

.012

.001

.019

.039

Collectiondate(1985)

4/30-5/:

4/30-5/25/3

4/30-5/25/3

--

4/30-5/25/3

--

--

--

5/2-3

--

4/29-305/1-2

—

--

4/29-305/1-3

CO2

(%)

•^•790

.029

.088

• 014

.013

--

.021

.033

--

--

—

.014

--

.035

.055

.046

.027

Carbon-14 activity and 613C values from individual probes obtainedU^?|by both KOH and MS methods are presented in table 3. In addition, volfc*Mgof COg gas collected and measured at 25°C and 1 atmosphere also aretabulated. ':

498

Tabte 2b. Carbon-dioxide concentrations from soil gas bygas-chromatograptiy method, borehole USW UZ 1

[CO2 concentrations are percent (%) by volume; dashes indicate no data)

ProbeNo.

1

2

3

4

5

6

7

8

9

10

U

12

13

14

15

Depth

(ft;

42

62

93

131

201

266

348

421

501

621

747

871

998

1,100

1,207

(m)

12.8

20.3

28.3

39.9

61.3

81.1

106. 1

128.3

152.7

189.3

227.7

265.5

304.2

335 3

367.9

11/9/83

0.024

<.005

.011

<.005

.019

.035

.032

.105

.024

.006

<.OO5

<.005

<.005

<.005

.020

l/IB/84

0.32

.01

.02

.004

.06

.08

.10

.06

.04

.06

.05

.003

<-003

.020

--

C02 (%)

3/12/84

0.67

.02

.016

.013

.085

.076

.13

.065

.059

.061

.063

.009

.006

.007

<.OO6

.006

--

--

on indicated

4/1-2/84'

i . 1

.03

,035

.02

.06

. 12

.075

.10

.05

.04

.025

<.004

m2

----

collection

4/9-10/84

0.83

.023

.026

.033

.24

.08

.07

.09

.05

.04

.026

<.004

.009

-

--

date

9/17/84

0.58

.037

.023

.077

.107

.07

.08

.06

.09

.04

.05

--

--

--

--

12/17/84

1.05

.045

.022

.079

.10

.026

.08

.09

.07

.04

.05

<.OO3

<.003

-.

--

5/4-6/85

1.641.37

.15

.01

.09

.04

--

.06

.06

.11

.06

. 11

.08

.10

<.02

<.01

.C2

.18

1 Samples analyzed onsite by Tracer Research, Inc.2 HD = not detectable.

Table 3. Su.wi.ry data for gas-phase CO, am) 6I3C and M C , borehole USW UZ-1

fErrors for o13C values = i 0.2fc; CO; X-voliime are weighted values; dashes indicate no data;blank indicates sample being analyzed)

ProbNo.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

11

e Depth

(ft) (•

S.

42

62

93

131

201

266

348

421

501

621

747

871

998

1,100

1,207

Air

.face

J2.8

20.3

28.3

39.9

61.3

81.1

106.1

128.3

152.7

189.3

227.7

265.5

304.2

335.3

367.9

. Collection' Date

--

(4/3O-5/2)B5

(4/30-5/4)85

(4/30-5/6)85

(4/30-5/4)B5

3/26-28/85

3/27-28/85

3/25-27/85

3/25-26/855/2-4/85

3/27-28/85

3/19-23/855/1-4/85

3/19-22/65

19-22; 24-26/85

3/19-25/85

1/19-22/85(4/29-5/1)85

Molecular-sieve

CO,(1 vol.

0.789

.043

.014

.024

.030

.016

.059

.009

.013

.024

.Oil

.045

.013

.003

.011

.039

.034

CO, vol

) (1)

--

5.680

1.230

1.18

.640

.430

.235

.430

.185

~.2IO

.800

.IBS

.104

.385

.540

method1

. fl13C(t)

--

-22.3

-26.2

-25.0

-23.8

-19.2

-23.3

-15. B

-20.1

-25.4

-11.0

-21.6

-21.0

(X

121.8

99.3

85.3

70.3

85.1

88.3

61.7

34.2

nodern)

--

t

t

•

1

±

±

±

±

1.8

!.7

2.33

3.3

4.01

2.5

l.B

1.5

Col lectionDale

4/05-10/84(4/29-5/6)854/06-11/84(4/30-5/6)85

4/2-7/B44/7-12/84(4/30-5/2)B55/5-6/85

4/1-10/84

4/1-10/84(4/30-5/6)85

4/1-10/84(4/3C-5/6)8S

4/1-10/84(4/30-5/6)85

4/I-J0/84

4/1-I0/B4(4/30-5/6)85

4/1-10/84

4/1-10/84

4/1-1O/S4

4/1-10/84

4/B-1O/84

4/9-10/84

KOH-method1

C02 vol(1)

1.871.716.403.60

8.113.28.96.9

0.87

0.881.18

1.103.0

2.62.30

1.50

2.702.. 60

3.70

2.J

1.7

!.G4

0.014

.004

. 8|:JC(t)

- 8 91- 8.56-16.63-14.62

-25.68-26.30-24.04-23.04

-14.58

-11.70-17.68

-19.96-20.79

-15.6B-17.87

-15.27

-14,79-16.59

-lS.'B

-15.13

-12.82

- 9.69

-18.09

—

(2 model

125.63124.43124.29123.39

120.120.120.120.

67.

62.79.

77.87.

77.72.

77.

76.72.

74.

73.

54.

49.

100.

73

55992375

42

5796

3757

3515

24

9888

62

23

02

94

4

.3

1tt1

•

i1t

t

tt

ti

1i

t

tt1

t

1

•

1

t

1.

0.1.

0.0.0.0.

0.

n)

01763855

66386462

.87

1.951.45

1.521.08

00

1

00

0

0

0

1

3.

2.

.79

.88

.48

.90

.74

.of

94

.90

.23

5«

•ol«cular-»lr»e nethod were aeasured by Geochron Laboratory, Kruger Enterprise, Inc.l «ic.rt a* alhetvite noted.

mr» M*i»r«d bj Southern HelhodUl University (SHU) radiocarbon laboratory,Ml«4

yMn»Ut«Uf, BbiMftlty •( Afliou, Tuiren, Arliona.

Carbon-14 activities: Several of the probes showed large changesbetween 1984 and 1985, and may still be changing. Comparing data by theKOH method, values for samples from probes 1 virtually were the same,those for probes 3 and 4 had large increases from 1984 to 1985, and thatfor probe 7 Lad a small decrease. For the three probes for which1985 data from both the KOH and MS methods are available, the results oftwo (probes 1 and 5) compare well, and for one (probe 3), the MS methodresulted in somewhat larger values. However, the magnitude of the differ-ence is not totally out of the range that might be expected from statis-tics, and more comparisons are needed.

The results of the three other samples (probes 2, 8, and 11) collec-ted by the MS method in 1985 indicate a substantially greater 1 4C0 2

activity than those from the same probes determined by the KOH method in1984. It is not known whether these discrepancies result from temporalchanges or from differences in the methods.

A plot of l2CC>2 and 1 4C0 2 mole fractions versus depth is presentedin figure 4 for samples collected in March and May 1985 by the MS method.The 14C02 mole fractions were calculated from a formula derived byThorstenson et al. (1903). The 14C02 mole fractions vere plotted at 10"

12

the scale of the 12C02 mole fractions. The data presented in figure 4 arecunsidered to be preliminary, because the gas concentrations continue tochange at the site, and the changes in 14C noted above may still beoccurring. However, one conclusive statement can be made regardingin the unsaturated zone. The mole fraction of 1 4C0 2 at probe 1 is sig-nificantly greater than that at the land surface or other depths. Thislarge value is the result of a series of atmospheric-nuclear tests con-ducted in the early i960'sv that introduced significant quantities of14C02 into the atmosphere, which subsequently were incorporated into thesoil gas through plant respiration. Therefore, this large value is a goodindicator of the penetration of gaseous C02 from the early 1960's to thatdepth. A general trend of rapidly decreasing 12C02 mole fractionsfollowed by small decreases with depth occurred as noted above, except inprobes 8, 11, and 15. Larger 12CO2 concentrations at probes 8 and 15 alsoare indicated by the GC method. More data are needed to confirm thesevariations with depth.

Carbon-13/Carbon-12 Ratios: Carbon 13/Carbon-12 ratios are pre-sented in table 3 for 12 of the 13 samples collected by the KOH method in1984, the 5 samples collected by the KOH method in 1985, and 12 of the 15samples collected by the MS method in 1985. Results for the samplescollected in 1984 have a tendency toward a fairly uniform profile dis-tributed about 613C values ranging from -15 to -17« (7 samples, with4 substantially heavier and 3 substantially lighter). Results forProbe 1 indicate exceptionally light values among these samples. Of thesamples collected by the KOH method in 1985, the 613C value for probe 1became heavier, and 6l3C values for the other 4 probes were somewhatj-o substantially lighter than those measured in 1984. The values for. * 3C in samples collected by the MS method tend to be lighter (-19 to: "26S;) but values for probes 10 and 13 are somewhat heavier.

501

LULULL

r-Q.HIQ

0

100

300

50O

700

900

1100

i3nn

3- ®A

O

9. 0 •

• 1 10

12o

13o

. o

14

2

• 5

a° 6

7i

0

0

11

• ° 15

i

0

^8

T i

.789 x 10'2

1.125 x•/•„•10'14

PROBE2 NUMBER

° 1 2 c O 2 '^ 14CO2»

• 12co2.A CO2»

3/85

3/85

4/85

4/85m

31

H 92

152

^ 213 -

277

4 335

3960.0 0.04 0.08 0.12

1 2CO 2 MOLE FRACTION, X10" 2 AND

1 4 C O 2 MOLE FRACTION, X1CT14

Figure 4. 12C02 and 14C02 mole fract ion versus depth

502

Comparisons of the fii3C values for samples collected by the twomethods are only possible for probss 1, 3 and 5. The 613C value for thesample collected by MS for probe 1 is somewhat heavier than that for thesample collected by KOH. For the other two probes, the 613C values forthe samples collected by MS are lighter by 6 to 7&. Discrepancies betweenthe methods are under investigation.

Water Vapor

Stable isotopic data of 6D and 6180 in water vapor collected fromindividual probes by the MS and cold-trap methods are given in table 4.In addition, the calculated in-situ, liquid-phase isotopic data of 6Dand 6180 are provided, assuming the vapor is in equilibrium with thewater phase at 25°C. As indicated in the table, the difference in iso-topic ratios of 6D in samples collected by the cold-trap and molecular-sieve methods is reasonably comparable for probes 7 and 8, whereas theyare significantly different for probes 1, 11, and 13. Moreover, thedirection and magnitude in 6D values determined by the two methods arenot consistent. Apparent differences in 6D by the two methods may bedue to fractionation of vapor during pumping from depth to the landsurface through several stages of condensation and evaporation in thesmall (0.118-in. or 3-mm) probe tubes. In any case, the overall cal-culated values of 6D. values are close to those in ground water at YuccaMountain and at nearoy areas (-97 to -110 &), obtained by Benson et al.(1983) and Claassen (1983). However, the 6:80 values determined by thetwo methods were very different in all cases, with the MS-method resultsbeing much heavier. This heavier tendency is attributed to the exchangeof oxygen atoms between the water and MS (SiC^) materials during degassingat 350°C.

Repeated sampling of water vapor by both methods or other improvedmethods will be continued to ensure that consistent values are obtained.These data also will be compared with isotopic data obtained by squeezingand centrifuging of pore fluids from cores to be collected at other areason Yucca Mountain.

Conclusions

Evaluation of Molecular-Sieve Method

The principal advantage of the MS method of CO2 gas and water-vaporsampling is its simple design and insured nonbreakable shipping betweenthe sampling site and the laboratory. For water-vapor collection, anexchange of oxygen atoms between the water and the molecular sieveoccurred during degassing and affected the oxygen-isotope values. Thehydrogen-isotopic data seem to be unaffected by exchange processes, andvalues were close to those in ground water at Yucca Mountain and atnearby areas. Thus, 6D data (and possibly tritium data) can be obtainedby the MS method simultaneously with CO2 samples, whereas the KOH methodcannot be used to collect both samples at the same time.

5U3

Table 4. Stable-1solope ratios la water vapor. t>«rt**k W & ni>1

[6 values are in parts per thousand (fc) relative to V-SMOW standard, error! for AD * » * mm6180 = ±0.2; CT = cold-trap method; HS = molecular-sieve method; dashes Indicate DO data)

Probe. No.

1

234567

8

9

1011

1213

14 1.15 1,

Depth(ft) (m)

42

6293131201266348

421

501

621747

871998

100207

12.8

20.328.339.961.381.1106.1

128.3

152.7

189.3227.7

265.5304.2

335.3367.9

Collectiondate

4/10/844/30/855/02/855/02/85

--5/02/853/26/854/10/843/25/854/10/843/25/853/27/853/25/853/26/855/02/853/23/855/01/853/21/854/29/853/20/854/10/843/21/853/21/853/19/854/29/855/01/85

Collectionmethod

fTHSMSHS--HSMSCTHSCTMSHSHSMSMSMScrMSHSHSCTMSMSHSHSMS

6DV

(fe)

-169.5-151.0-173.5-170.5

---161.0-185.0-187.0-188.5-170.0-175.5-172.0-189.0-194.0-174.5-176.5-175.5-191.0-159.0-164.0-188.0-176.5-172.0-172.0-157.0-142.5

6180V

(fc)

-19.7- 6.4-12.3-13.9

--- 7.5-10.6-23.6-12.6-22.0- 9.5- 7.0-15.6-13.4-14.4-13.8-22.7-14.5- 5.7- 7.3-24.7-14.7-17.9-14.4- 8.0-13.9

60.l

(fc)

-108.0- 88.2-112.3-109.1

—- 98.9-124.7-126.8-128.4-108.6-114.5-110.7-129.0-134.4-113.4-115.6-114.5-131.1- 96.8-102.1-127.9-115.6-110.7-110.7- 94.6- 790

J(b)

-10.7+ 2.7- 3.2- 4.8

—+ 1.6- 1.5-14.6- 3.5

-n.o- 0.4+ 2.1- 6.5- 4.3- 5.3- 4.7-13.7- 5.4+ 3.4+ 1.8-15.7- 5.6- 8.9- 5.3+ 1.1- 4.8

Note: * 6D., 6l8O are calculated liquid-phase isotopic data, assuming water-vapor equilibriuin at 25°C.

Thorstenson, D.C., E.P. Weeks, Herbert Haas and D.W. Fisher. 1983.Distribution of gaseous l2C02,

1 3C0 2 and 14C02 in the sub-soil

unsaturated zone of the Western U.S. Great Plains. Radiocarbon,v. 25, no. 2, pp. 315-346.

Wbitfield, M.S., 1985. Vacuum drilling of unsaturated tuff at a potentialradioactive waste repository, Yucca Mountain, Nevada, I_n Proceeding,National Water Well Association Conference on Characterization andMonitoring of the Vadose (unsaturated) Zone, Denver, Colorado,November 19-21, 1985.

Winograd, I.J. 1972. Near surface storage of solidified high-levelradioactive waste in thick (400-2,000 feet) unsaturated 2ores in theSouthwest. Geological Society of America Abstracts with Program, v.4, no. 7, pp. 708-709.

Winograd, I.J. 1974. Radioactive waste storage in the arid zone.Transactions of the American Geophysical Union (EOS), v. 55, no. 10,pp. 884-894.

Winograd, I.J. 1981. Radioactive waste disposal in thick unsaturatedzones. Science, v. 212, no. 4502, pp. 1457-1464.

Yang, I.C. 1983- Evaluation of direct-precipitation and gas-evolutionmethods for radiocarbon dating of ground water. Radiocarbon, v. 25,no. 2, pp. 511-518.

Biographical Sketch

A native of Taiwan, Dr. In Che Yang received his bachelor of sciencedegree in chemical engineering from National Taiwan University, master ofscience in nuclear chemistry (1966) from Carleton University, Canada, anddoctor of philosophy degree in isotope geochemistry (1971) from theUniversity of Washington. He worked at the Department of Geology, Uni-versity of Washington as a post-doctor (1971-73), Research Assistant(1973-75), and Research Assistant Professor (1975-78) after graduation.Dr. Yang joined the U.S. Geological Survey in April 1978. He vas sectioachief of the Radiochemical Section (1980-84), and joined the NuclearHydrology Program (1984 to present). He is currently a Project Chief ioHydrochemistry. His address - : U.S. Geological Survey, Box 25046, MS416, Denver Federal Center, Denver, CO 80225.

Edwin P. Weeks received his bachelor of science degree in geologicalengineering from Colorado School of Mines, Golden, Colorado, in 1958. ••has worked in various aspects of ground-water research at the Water ^Resources Division of the U.S. Geological Survey from 1958 to present.Since 1958, he has served as the chief of various projects involvingstudies of the impacts of irrigation on ground-water systems, developof air-permeability techniques to characterize unsaturated zones,gation of artificial recharge methods, studies of anisothermaltransport and gaseous diffusion in the unsaturated zone, andof evapotranspiration. He has also taught courses at the U.S.Survey Training Center in various ground-water related subjects,currently Chief of the Unsaturated Zone Field Studies in Denver,His address is: U.S. Geological Survey, Box 25046, MS 413, 1[)eaVet^Center, Denver, CO 80225.

(Biographical information for H.H. Haas and D.C. Thorstenson w«»available at time of printing.)

506

Carbon-Dioxide and Carbon-Isotope Concentrations

The CO2 concentrations measured by MS and GC methods were notconsistent, possibly because of continued fluctuations of unknown origin.In theory, the MS method should have fewer fluctuations by integratinglong-term (4 to 5 d) average values.

The 14C data indicate general agreement between the molecular-sieveand KOH methods for comparable samples, and significant differences forS13C values between samples. More data are needed to resolve thesedifferences. Post-bomb 14C was detected to a depth of 40 ft (12.2 m)below land surface, and less than pre-bomb activity was measured at andbelow a depth of 60-ft (18.3 m).

References

Benson, L.V., J.H. Robison, R.K. Blankunnagel and A.E. Ogard.1983. Chemical composition of ground water and the locations ofpermeable zones in the Yucca Mountain Jjrea, Nevada. U.S.Geological Survey Open-File Report 83-854: 19 pp.

Claassen, H.C. 1985. Sources and mechanisms of recharge for groundwater in the west-central Amargosa desert, Nevada--a geochemicalinterpretation. U.S. Geological Survey Professional Paper 712-F:61 pp.

U.S. Department of Energy. 1984. General Guidelines for theRecommendation of Sites for the Nuclear Waste Repositories—FinalSiting Guidelines. Federal Register, 10CFR Part 960.4-2-1(b)(1)v. if9, no. 236.

U.S. Nuclear Regulatory Commission. June 30, 1983. Disposal of High-I.evel Radioactive Waste in Geologic Repositorxss—LicensingProcedure, Siting Criteria. Federal Register, 10CFR Part 60.122,(b)(2)(iv), v. 49, no.33.

U.S. Nuclear Regulatory Commission. February 16, 1984. Disposal ofHigh-Level Radioactive Waste in Geologic Repositories—LicensingProcedure, Siting Criteria, Proposed Rules. Federal Register, 10CFRPart 60.122(c)(24). v. 49, no. 33.

Haas, H.H., D.V. Fisher, D.C. Thorstensen and E.P. Weeks. 1983.13C02 and

14CC>2 measurements on soil atmosphere sampled in thesub-surface unsaturated zone in the western great plains of theU.S. Radiocarbon, v. 25, no. 2, pp. 391-314.

Montazer, Parviz, and W.£. Wilson. 1984. Conceptual hydrologic model offlow in the unsaturated zone, Yucca Mountain, Nevada. U.S. Geo-logical Survey Water-Resources Investigations Report 84-4345: 55 pp.

;Hontazer, Parviz, E.P. Weeks, Falah Thainir, S.N. Yard amd P.B. Hofrichter,r 1985. Monitoring Vadose zone in fractured tuff. Yucca Mountain,t Nevada. K* Proceedings, National Water Well Association Conferencet °n Characterization and Monitoring of the Vadose (unsaturated) Zone,V Denver, Colorado, November 19-21, 1985.j» °seboom, E.H. 1983. Disposal of high-level nuclear waste above the

water table in arid regions. U.S. Geological Survey Circular 903:21 pp.

505